v0.6.0

Adds a custom vocabulary to the schema validator to support OpenAPI 3.0 keywords and provide proper nullable validation for 3.1 (not allowed) and 3.0 (allowed) as well as other OpenAPI specific keywords like discriminator

daveshanley/vacuum#710
daveshanley/vacuum#603

Also adds Jackson-style scalar coercion (string->boolean/number) configuration options.